Bug 1472805 - silence more clang-cl warnings; r=chmanchester
clang seems to have decided that a new warning,
-Wused-but-marked-unused, is useful. For the purposes of our codebase,
where we add __attribute__((unused)) explicitly to avoid warnings about
unused things, this new warning is useless. The new warning is also
quite noisy, given that we have unused things in commonly-used headers
like Assertions.h. So disable the warning.